The invention relates to a tank or a tank system, in particular to a tank for transporting cryogenic liquids in tankers.
The transport of liquid gasses by water has in the meantime developed into a sophisticated branch of industry, which has at its disposal a large fleet of tankers, a network of export and import terminals, along with a wealth of knowledge and experience from the various individuals participating in the processes. Gas tankers are equipped with specific features that they share with other tankers used for conveying bulk liquid cargoes, such as oil and chemicals.
Liquid tankers today represent a flexible alternative to the transport of liquid gas in pipelines, and are used for the maritime transport of liquefied gases as bulk cargo in fixedly installed cargo tanks. Apart from technical gases, transported gases primarily include liquefied natural gases (LNG—liquefied natural gas) and liquefied gases (LPG—liquefied petroleum gas).
Ship tanks for liquefied gases can basically be divided into two categories. There are the “non-freestanding tanks,” such as the prismatic membrane tanks, and the “freestanding tanks,” for example the spherical MOSS tanks. The membrane tanks are characterized by the fact that they adapt to the outer ship geometry and—by comparison to spherical MOSS tanks—better utilize the available space. In comparison to the described MOSS system, this makes these ships lighter or smaller given the same load. However, the disadvantage here is that partial loads, e.g., between a 10% and 70% fill level, are not permissible, since the arising sloshing movements of the liquid can damage the tank walls. In addition, the pressurization possible on the tank is low. Cryogenic liquids can thus be transported only at approximately an ambient pressure. As opposed to when being stored in spherical tanks, they are therefore not supercooled in their saturation state during transport, and thus impaired by elevated evaporation losses. Common evaporation rates measure approx. 0.15% per day. Minimizing evaporation losses is the primary objective for this transport container. The efficiency of liquid transport and potentially the burden on the environment depend heavily on the evaporation losses.
FIGS. 1 and 2 describe a conventional membrane tank 1, which commonly exhibits a prismatic basic shape 2. The membrane is constructed in such a way that it can absorb higher temperature-induced expansions owing to the material properties and expediently arranged folds and beads. For example, temperatures of −164° C. to −161° C. arise during the transport of liquefied gases given LNG as the cargo. The temperature inside the tank can reach as high as 40° C. or more while the ship is docked at a shipyard.
A line 3 lying on the interior—the so-called “pump tower”—is used to fill and empty the membrane tank 1. As may be gleaned from FIG. 2, a typical liquefied gas tanker consists of a series of several prismatic tanks 1, which are separated from each other by intermediate areas 4 called “cofferdams.” The temperature in these intermediate areas 4 is generally warm by comparison to the supercooled liquids.
Due to their prismatic shape 2, these membrane tanks 1 are heavily influenced by the sloshing movements of the liquids, so that damages can arise, in particular in proximity to corners and edges, where especially high sloshing forces can arise.
It is generally known that sloshing forces can be reduced by additionally installing various baffles. For example, WO 2011/129770 A1 describes a system in which the free liquid surface is stabilized by incorporating an additional plate. WO 2006/014301 A1 focuses on the same problem by incorporating systems for reducing the sloshing forces in the corners of the tanks.
In general, such damping systems are characterized by the fact that they are applied to the insulating wall structure to elevate the evaporation rates. In actual fact, the heat bridges this produces are not conducive to the insulating effect of the tank. In addition, it must be considered that the filling line 3 of the tank 1 allows additional heat to penetrate.
A majority of the heat penetrating into the tank does so via the tank cover 5, which generally is distinctly warmer than the liquid stored in the tank. If these surfaces are briefly wetted with liquid by sloshing movements, this increases the evaporation rate.
Additional losses due to thermal radiation as well as thermal conduction through the gas result in elevated evaporation losses. For purposes of optimized storage, use is therefore made of land-bound storage tanks 6 for cryogenic fuels with suspended false ceilings 7, for example which are suspended on ropes 8. False ceilings 7 significantly reduce the evaporation rate. Such a tank is depicted on FIG. 3. In the type at hand, however, its implementation is not suitable in a ship tank for transporting liquids in membrane tanks.